This invention generally relates to electrical circuits and, more particularly, to electrical circuits for rotating cartesian coordinate signals.
A phase rotator is a device having two input ports for receiving cartesian coordinate signals x, y and two output ports for cartesian coordinate signals x', y'. The transfer characteristics of a phase rotator are: EQU x'=x cos .theta.-y sin .theta. Eq. 1 EQU y'=y cos .theta.+x sin .theta. Eq. 2
where .theta. is a pre-determined angle of rotation.
Phase rotators are commonly used in processing the signals measured by eddy current inspection instruments when non-destructively testing for flaws, discontinuities, and cracks in metallic materials. Curve A in FIG. 1 illustrates the signal response pattern from a typical single frequency, eddy current inspection probe. The probe has two output data signals and when this data is displayed on an x-y oscilloscope, a Lissajou pattern results.
Typically, phase rotators are used to rotate the signal response pattern as it is displayed in cartesian coordinates and to remove unwanted test parameters from the data signals. One such parameter is the probe-to-specimen spacing between the eddy current probe and the object being measured. This spacing induces an error that appears as a straight line on an x, y cartesian presentation. A phase rotator is used to rotate the signals until the error is fully contained on either the x or the y axis. The output signal displayed along the other axis is then free of the error and is used as the measured parameter.
A plurality of phase rotators are commonly used in connection with a multiple frequency eddy current inspection probe to process the measured multiple frequency data. A plurality of phase rotators can process multifrequency data by treating this data as an N-dimensional vector space problem. The multiple rotators perform a process similar to the simultaneous solution of multiple independent equations.
Although most eddy current inspection instruments today have phase rotators, these rotators incorporate sine and cosine potentiometers and operational amplifiers to perform the transfer function, Equations 1 and 2. These potentiometers typically require manual adjustment and inject noise into the output signals. Further, these circuits lack high resolution and limit the amount of data which can be obtained by this type of signal processing.